Developing a new source of water resources has emerged as an urgent facing problem due to recent serious pollution of water quality environments and water shortage. Studies on the pollution of water quality environments aim for high-quality residential and industrial water, and treatment of various domestic sewage and industrial wastewater, and interests in water treatment processes using a separation membrane having an advantage of energy saving have been rising. Further, accelerated reinforcement on environment regulations is expected to advance activation of separation membrane technologies. It is difficult for traditional water treatment processes to satisfy the tightened regulations, but separation membrane technologies secure excellent treatment efficiency and stable treatment, and accordingly, are expected to become a leading technology in the field of water treatment in the future.
Liquid separation is divided into micro filtration, ultra filtration, nano filtration, reverse osmosis, stannizing, active transport, electrodialysis, and the like, depending on the pore of the membrane. Among them, the reverse osmosis method refers to a process of performing desalination work by using a semi-permeable membrane which permeates water, but shows impermeability to salts, and when high pressure water in which salts are dissolved is introduced into one surface of the semi-permeable membrane, pure water from which salts have been removed is released into the other surface at low pressure.
Recently, water on a scale of approximately one billion gal/day has been subjected to a desalination process through the reverse osmosis method, and since a first desalination process using reverse osmosis was published in the 1930's, numerous studies on semi-permeable membrane materials in this field have been carried out. Among them, what is mainly used due to the commercial success of the reverse osmosis method is a cellulose-based asymmetric membrane and a polyamide-based composite membrane. Cellulose-based membranes developed in the initial phase of the reverse osmosis membrane have recently tended to be rarely used due to various disadvantages in that the operable pH range is narrow, the membranes are deformed at high temperature, lots of costs required for operation are needed due to the use of high pressure, the membranes are vulnerable to microbes, and the like.
Meanwhile, polyamide-based composite membranes are manufactured by a method of forming a polysulfone layer on a non-woven fabric to form a microporous support, immersing the microporous support in an aqueous solution of m-phenylene diamine (hereinafter, referred to as mPD) to form an mPD layer, immersing the microporous support again in an organic solvent of trimesoyl chloride (hereinafter, referred to as TMC) or coating the microporous support with the organic solvent to bring the mPD layer into contact with TMC, and performing an interfacial polymerization to form a polyamide layer. By bringing a non-polar solution into contact with a polar solution, the polymerization occurs only at the interface thereof, and accordingly, a polyamide layer having a very small thickness is formed. The polyamide-based composite membranes are highly stable against a change in pH, can be operated at low pressure, and have an excellent salt rejection rate, as compared to the existing cellulose-based asymmetric membranes, and accordingly, the polyamide-based composite membranes are currently in the mainstream of water treatment separation membranes.
Meanwhile, there are several conditions required for commercially using the water treatment separation membranes, and one of the conditions is that the water treatment separation membranes need to have a high salt rejection rate. The salt rejection rate commercially required for the water treatment separation membrane is at least 97% or more based on the brackish water. Examples of the other important properties of the water treatment separation membranes include an ability to allow a relatively large amount of water to pass through the membrane even at relatively low pressure, that is, high flux characteristics. However, since the salt rejection rate and the permeation flux characteristics have conflicting properties, there are actually many difficulties in manufacturing a water treatment separation membrane having excellent salt rejection rate and permeation flux.